Other vulnerable low-lying parts of the U.S. coastline, such as in Florida and Maryland, are sinking by one to two feet every 100 years. But in the updated report, the only spots marked in red are in Louisiana.

"Louisiana has the highest rate in the country," William Sweet, an oceanographer for the National Oceanic and Atmospheric Administration, said Tuesday (April 18). "And that really poses a problem."

Grand Isle is losing about an inch every three years, according to data from the National Oceanic and Atmospheric Administration (Graph by NOAA).

The state's sinking coastline means the loss of marshlands that protect populated areas from storm surges and flooding. "You're losing a lot of land and a lot of buffer," Sweet said.

Why is Louisiana's coast shrinking so fast? Climate change and other factors are in play, but the biggest single reason appears to be the Mississippi River, Sweet said. Much of the state's coastline is made of the fine grit and sand flowing out of the river. Relatively soft and fluffy, this sediment compacts as it settles over the years. It also erodes easily, giving Louisiana a much faster rate of subsidence.

The National Oceanic and Atmospheric Administration monitors sea level rise at other river deltas in the U.S., but none are like the Mississippi Delta. "It's hard to find a sister system that can compare to the scale and magnitude of the Mississippi," Sweet said.